Lithography is used in the manufacture of semiconductor chips. Lithography, or more particularly photolithography, involves projecting one or more images of a reticle or semiconductor circuit mask onto a photosensitive substrate of a wafer. The wafer is then processed to form one or more circuits. As the art of semiconductor chip manufacturing progresses and the size of semiconductor devices become smaller, there is a need for improving line width control in photolithography devices.
Large semiconductor chips are typically manufactured using a step-and scan lithography device. A step-and-scan lithography device operates by scanning a typically rectangular illumination field defined by an illumination system over a reticle having a circuit pattern thereon. A step-and-scan lithography device is used to manufacture large semiconductor chips, in part, because the size of a semiconductor chip that can be manufactured using a step-and-scan lithography device is not limited to the size of the device's projection optics.
A method and system for improving line width control, for example, in a step-and-scan lithography device, is described by McCullough et al. in U.S. application Ser. No. 09/599,383, filed Jun. 22, 2000, “Illumination System With Spatially Controllable Partial Coherence Compensating For Linewidth Variances In A Photolithography System,” which is incorporated in its entirety herein by reference. McCullough et al. describe using a custom-designed optical element, such as a microlens array or a diffractive optical element, to control the partial coherence of an illumination system of a lithography device and thereby compensate for line width variances in the lithography device. The custom-designed optical element described by McCullough et al. is designed to compensate for predetermined horizontal and vertical biases associated with a particular lithography device. A limitation of the method of McCullough et al., however, is that it is typically an expensive and time-consuming process to design and manufacture the custom-designed optical element described by McCullough et al. Thus, the custom-designed optical element described by McCullough et al. cannot be readily adjusted, for example, as the horizontal and vertical biases associated with a particular lithography device change with time.
Other types of lithography devices such as step-and-repeat lithography devices and field-stitching lithography devices also exhibit horizontal and vertical biases that cause line width variances. Compensating for the horizontal and vertical biases in these lithography devices and improving line width control is just as important as compensating for horizontal and vertical biases and improving line width control in a step-and-scan lithography device.
What is needed is a system and method for controlling line width variations in a lithography device that overcomes the limitations described above.